340 
PROFESSOR W. N. HARTLEY ON SPECTRUM PHOTOGRAPHY 
parts of magnesium in 100,000 volumes of solution. The mineral, therefore, was 
considered to contain 1 per cent, of magnesium. The comparison of the photographs 
was made by three independent observers who each gave the same figures* from two 
different series of photographs. By simple inspection of the first photograph the 
calcium was seen to be present to the extent of something between 30 and 40 per 
cent, of the mineral. The more precise estimation of this mineral presented some 
difficulties, both on account of the mineral containing this element as its chief con¬ 
stituent, and by reason of the fact that dust floating in the air is apt to contaminate 
the electrodes with calcium, but not with magnesium. The hydrochloric acid used 
likewise always shows this substance to be present. 
The estimation of calcium .—A series of solutions of calcium chloride were made 
containing the following proportions of calcium : xwoth, I' o'W th, 3 W 0 oth, -y^gth, 
_1_|L _1_t 1 1 -2- i- 1 1 -3-| U -4-|U-5-U -3-U _4_4.1. pTirl 
5000" ) 10,000 Lil > lO.OOO 1,11 ! 10,000 uu 5 10,000 111 ) 10,000 LU ) iOO,OOO UI J 100,000 lu > cliiU 
10 , 000 ' 
_5_IP 
1 oo,ooo ul * 
The first photograph of the mineral, which was taken from a solution containing 
1 gram dissolved in hydrochloric acid and made up to 1 litre, exhibited a spectrum, 
the length and strength of the lines in which indicated between 1 and 2 parts of calcium 
in 4000 of liquid. The solution of the mineral containing 1 in 10,000 corresponded 
very closely with 3 in 100,000 of the calcium solution, or about 30 per cent. 
On account of the strength of the calcium lines and the occasional vitiation of the 
results by calcium in floating dust, it was deemed advisable to shorten the period of 
exposure from two minutes to half a minute. 
The next photograph showed that 4 in 100,000 of calcium solution was stronger 
than 1 in 10,000 of the mineral. There is therefore less than 40 and more than 30 per 
cent, calcium in the mineral. 
Photographs of the calcium solution were then taken containing 3*5 centims., 3'6 
centims., 3'7 centims., 3'8 centims., and 3‘9 centims. of the standard solution in 
100 centims., or 3’5 parts per 100,000 and so on. 
Photographs were then taken with a quarter and with half a minute’s exposure, which 
was found sufficient; even five seconds gave very fair indications of the calcium lines. 
The solution of the mineral containing 1 - 0,0 o o th was then treated in the same way; 
the results are as follows :— 
* To speak more particularly, tlie photographs were taken by Mr. Templeton, a student in the Royal 
College of Science, and his results were checked by Mr. W. E. Adeney, the Assistant Chemist, and by 
myself. A second series of photographs w r as also taken by myself. 
